1. Field of the Invention
The present invention generally relates to systems and methods for adjustment of radio frequency (RF) power amplifiers (PAs), and more particularly to a system and method for dynamic drain voltage adjustment to control linearity, output power, and efficiency in RF power amplifiers.
2. Discussion of the Background
Cellular phone systems rely on wireless communication between handsets and Basestations. A Remote Radiohead (RRH) is a transceiver subsystem that is used in conjunction with a cellular base station. The telephony and data traffic is sent to the RRH via an electrical or optical connection. The RRH generates one or more carrier frequencies and modulates them with the traffic signal. The modulated carries are amplified to the desired level in a power amplifier (PA). The PA unit is a self-standing functional-block that is formed by several amplifier stages. In a typical system, the PA is used in conjunction with digital predistortion or feedforward correction to achieve the linearity requirements. However, PA peak power capability will still set the performance limit for a given average power requirement and the system efficiency.
Power amplifier efficiency has been and continues to be the topic of intense research in electronic/electrical engineering discipline. The efficiency requirement is more stringent in battery operated systems and also where there are limitations in cooling an amplifier unit. A familiar example is the aforementioned RRHs, especially tower mounted units, in which convection cooling (i.e., fanless) is preferred for reliability reasons. Such systems that serve a large cell in mobile communications infrastructure require a relatively large radio frequency (RF) output power and hence a relatively high power amplifier, which means larger size. This limited convection cooling combined with a high power PA is a major issue driving PA efficiency for RHH applications. Moreover, due to the nature of modulation scheme used in such systems and the number of carriers used, the composite signal exhibits a large Peak to Average signal Ratio (PAR). This means the instantaneous signal power can peak to values much larger (e.g., 3-10 dB) than the average output power. This in turn means that the amplifier will have to be sized to handle much higher powers than the average output requirements with implications for the PA efficiency (i.e., the higher the PAR, the lower the efficiency). The heat generated by running a PA at low efficiency, but high power, prohibits their application in convectively cooled systems.
There are many power amplifier design techniques to address some of above problems, such as designs using feedback amplifiers, designs using class AB, D, E, and F amplifiers, and designs using envelope tracking amplifiers. However, each of the known techniques has some advantages and disadvantages. For example, the feedback amplifier has improved linearity, but the bandwidth is limited. The Class AB amplifiers have very wide bandwidth and good linearity, but high PAR efficiency is poor. The class D, E, and F amplifiers have very high efficiency, but suffer from very poor linearity.